CHEMISTRY COMMUNITY

Sabrina Dunbar 1I

You do factor in the heat of fusion in these questions. You need to account for the heat required to melt the ice and then to raise the liquid water to the final temperature. Hopefully this helps.

Sabrina Dunbar 1I

A substitution reaction occurs when an atom (or group of atoms) is replaced by another atom (or group of atoms). This kind of reaction is common in organic molecules with a leaving group and a nucleophile that replaces the leaving group in the reaction. Because Dr. Lavelle went over it in class, I w...

Sabrina Dunbar 1I

HI, when we are given a table of values like the one given in Karen Leung's Worksheet 7 titled Kinetics Part 1, how to we incorporate coefficients from the chemical reaction equation into our calculations? The equation given is 2A+B+3C=2D+F. The table given is the same as others we have seen in the ...

Sabrina Dunbar 1I

Thanks Justin. I solved it the same way you did initially but upon checking the solutions manual it proposed a three step process and used Hess's Law to solve instead of ∆H values. Your explanation was very helpful, thank you!

Sabrina Dunbar 1I

How would we know to use oxygen in the proposed mechanisms for the hydrogenation of ethyne to ethane? I understand the process of Hess' Law used in the solutions manual but am confused on how the book knew to use oxygens in the separate steps.

Sabrina Dunbar 1I

No, there is not a periodic trend we can rely on for this type of deduction. You need the reduction half reactions to determine which element in a reaction has a greater oxidizing or reducing power.

Sabrina Dunbar 1I

The rate law is determined by the slow step, which in this problem is NO+Br2-->NOBr2. The rate law based on this step in the process is rate=k[NO][Br2] and the molecularity is two because there is one molecule of each NO and Br2 colliding to create the intermediate NOBr2. Hopefully this helps!

Sabrina Dunbar 1I

In class on Friday, Dr. Lavelle showed us the Arrhenius equation, which is K=Ae^Ea/RT. This is another way of expressing the constant K and finding other components of the equation quantitatively. That is the equation used in this problem to find K.

Sabrina Dunbar 1I

The unique rate is the instantaneous rate found using the ratios like (1/2)(dA/dt).

Sabrina Dunbar 1I

The units for k change with the different orders of reactions we solve. Homework question 15.9 shows this concept in a question form nicely. Because the units for time and concentration are constant, the units for k must change when we use natural logs versus 1/concentration and so forth.

Sabrina Dunbar 1I

That will not be on test 3, as we will only be tested on 15.1-15.6! We will go over that for the final I believe but you do not need to know that for the test this week. Hopefully that answered your question.

Sabrina Dunbar 1I

The unique rate is the instantaneous rate of change at any time t during the reaction. Unlike the average rate, which takes into account all of the times at which the reaction proceeds and averages it, the instantaneous rate is specific to a time and therefore it generates a different k value. The g...

Sabrina Dunbar 1I

Can someone explain to me how we incorporate the coefficient of 2 from the reactant A into figuring out k? I got the correct answer for parts (a) and (b), but I am lost on which equation to use for part (c). Any guidance would be greatly appreciated!

Sabrina Dunbar 1I

I am assuming you mean 15.13 or else I would be doing the wrong problem! First, you have to convert the grams of reactants given into moles, and then from moles to their respective molarity values. You get these values to be 0.347 mol/L and 0.001 mol/L. Then, multiple these values and the given k to...

Sabrina Dunbar 1I

In 15.3, the entire rate law for the reactant should have a negative sign in front of it. However, because we find the change in concentration as final-initial, 320mmol-450mmol=-130mmol. When fully written out, rate=∆concentration/∆t=-(320mmol-450mmol)/20s=+6.5x10^-3. The negative signs cancel out t...

Sabrina Dunbar 1I

Hi, I am confused about what equation or set of equations to use when you have a problem with changing temperature and volume. An example of this would be from Karen Leung's worksheet titled 2nd and 3rd Laws of Thermodynamics, and it is the last part of question 6. Another balloon expands from 3.3L ...

Sabrina Dunbar 1I

You will be able to tell which one you can use by the values you are given. If you have the ∆Formation for the reactants and the products given, then use that method for solving for ∆G. Ig you are given the proper values to calculate ∆G using enthalpy and entropy, then that method would be the best ...

Sabrina Dunbar 1I

Residual/positional entropy is the entropy calculated at a constant temperature, independent of the enthalpy value of the system. The other entropy, thermal entropy, incorporates the ∆H value because there is a change in temperature of the system. Boltzmann's equation does not account for temperatur...

Sabrina Dunbar 1I

The surroundings are everything else in the universe except for the system. So with you interpretation the system as just being the reaction, then the surroundings are equal to the entire universe minus that reaction system.

Sabrina Dunbar 1I

W is the degeneracy, or the number of ways of achieving a given entropy value. If you know the desired entropy, you can use the Boltzmann equation to plug in the value you are given along with Boltzmann's constant to solve for W. In this form of solving for W, you would have to use your previous kno...

Sabrina Dunbar 1I

Standard conditions are 1.00 atm and 298 K, which is 25ºC.

Sabrina Dunbar 1I

Using the equation ∆S=q/T, we can substitute ∆Hº for q, and the the appendix provides the value for that variable. Because the given ∆Hº is per mole, multiply the numerator by the number of moles in the sample to then produce the correct value. Hopefully this will give you a good jumping off point t...

Sabrina Dunbar 1I

For this question in particular, you are given all of the information necessary to calculate the number of moles using the ideal gas law, PV=nRT. n turns out to not be 1.00 mol, so here the solution would be incorrect if 1.00 mol was assumed to be the value of n. I think a general rule would be that...

Sabrina Dunbar 1I

The term ∆S means the change in entropy, not just entropy itself. By increasing the temperature, the change in entropy is less than if the reaction were to occur at a lower temperature. The larger T value in the denominator makes this ∆S value less because as you said, the two are inversely related....

Sabrina Dunbar 1I

The Kb that we are using here is not the same Kb as in acids and bases calculations. Here, we are using Boltzmann's constant, k = 1.38 x 10–23 J·K-1, to stand for Kb in free energy and entropy calculations. Hopefully this answers your question.

Sabrina Dunbar 1I

I got the correct answer taking the natural log of (.500 atm/15 atm) and then multiplying it by the constant value R and 1.5 as the number of moles of neon. I will include the substituted values that I used below. Maybe it was a simple calculation error? ∆S=(1.5 mol)(8.314 J/Kmol)(ln(.500 atm/15 atm...

Sabrina Dunbar 1I

∆U=∆H only when the work of the system is zero. As Dr. Lavelle was saying, this is almost always the case in biological systems because there is little to no work done on biological systems. In reference to the equation for determining the change in internal energy, ∆U=∆H+w, if w=0, then the equatio...

Sabrina Dunbar 1I

Another example of an isolated system could be a high grade thermos bottle like the question 8.1 shows us. The calorimeter is definitely the most popular and I would expect that to the the system used on a test dealing with isolated systems.

Sabrina Dunbar 1I

To my knowledge, tests are written by Dr. Lavelle, not the TAs. However, not all of the tests are exactly the same. They are all of equal difficulty but differing in the specific questions asked.

Sabrina Dunbar 1I

Enthalpy is the amount of heat released or absorbed at constant pressure and it is written as H. When this H is changed between the products and the reactants in a chemical equation it becomes ∆H, meaning it is the change in enthalpy. This differs from the internal energy of the system greatly. The ...

Sabrina Dunbar 1I

An intensive property is a property that depends on the amount of substance that is being measured. Extensive properties lack this substance measurement. We use the specific heat capacity as an intensive property because we can measure how much a a substance is involved in a calorimetry experiment a...

Sabrina Dunbar 1I

Yes, that is all of the material that we have learned so far this quarter.

Sabrina Dunbar 1I

The delta just means that it is the change in enthalpy, so H is enthalpy. However, that H value is to as important to us as the change in that value which you already stated is equal to q. Enthalpy is the measure of heat released or absorbed by a system at constant pressure, and the delta just makes...

Sabrina Dunbar 1I

Why do we want to use extensive properties instead of intensive properties if possible? What is we are only given intensive to use for a calculation? Thanks!

Sabrina Dunbar 1I

Then, it would no longer be the enthalpy, it would just be the heat related or absorbed at the differing pressures. Enthaply has to be measured at a constant pressure.

Sabrina Dunbar 1I

Yes that is exactly when you would use that equation:) The sum of pH and pOH is 14.

Sabrina Dunbar 1I

There are two different naming systems used for ligands. They can both be found on the Naming Coordination Compounds sheet that Dr. Lavelle has posted on his website. Ligands with the -ido endings come from the IUPAC form of the name and the -ido is interchangeable with -o. Just stick to one form wh...

Sabrina Dunbar 1I

HCl is stronger than HF because of their size, so in turn that means that it is indeed because of their bond length. Fluorine atoms are smaller than chlorine atoms and that makes the bonds between the hydrogen atom and the fluorine atom shorter and stronger than that of a hydrogen and a chlorine ato...

Sabrina Dunbar 1I

This molecule HClO2 is an acid, so it releases an H+ proton in solution. After that dissociation occurs, you would be left with a molecule with an O as the central atoms when it really should be a Cl. Physically, the much more stable arrangement of the molecules has the two oxygens bonded to a centr...

Sabrina Dunbar 1I

30. A mixture of 2.5 moles H2O and 100 g of C are placed in a 50 L container and allowed to come to equilibrium subject to the following reaction: C(s) + H2O (g) ⇌ CO (g) + H2 (g). The equilibrium concentration of hydrogen is found to be [H2] = 0.040 M. Calculate the equilibrium constant Kc of this ...

Sabrina Dunbar 1I

I will use the example of an equilibrium equation A+B<-->C. The Kc for this forward reaction is Kc=C/A*B. The reverse reaction of this equilibrium system is C<-->A+B. In this reverse reaction, Kc=A*B/C. The two Kc equations have the numerator and the denominator flipped because the reactants and the...

Sabrina Dunbar 1I

Cobaltate is used when the compound is an ion with a net negative charge while cobalt is used when the molecule has a net positive charge. Other TMs like iron can become ferrate when they have a net negative charge. Hope this helps.

Sabrina Dunbar 1I

In terms of naming compounds with transition metals in them, the oxidations of transition metals is never zero (I think) and therefore you would never have a negative Roman numeral. The Roman numeral corresponds to the TW, so as the TM oxidation states are positive, the Roman numerals are also posit...

Sabrina Dunbar 1I

When you draw out the lewis structure for the molecule, you can see that the dipoles do not cancel out. Unlike CH4 where the bonded atoms are all the same, methanol has three Hs and one OH. Therefore, the dipoles will not cancel because there are differences in their strengths.

Sabrina Dunbar 1I

Once the equation is balanced, it becomes 4C4H10+26O2-->16CO2+20H20. There are 30 moles of reactants and 36 moles of products. The difference between total moles of products and total moles of reactants gives us the net moles of gas produced (36-30=6). Therefore, the answer is 6 moles of net gas pro...

Sabrina Dunbar 1I

There is an error in the solutions manual, so this is what Dr. Lavelle has posted on his website in reference to this error: 2.93 In the picture, it shows A (smaller atom) + B (larger atom) --> C (larger ion) + D (smaller ion) The solution manual says that A=Na and B=Cl, and it references Figure 2.2...

Sabrina Dunbar 1I

The electronegativity values help us to see what type of bond is formed between elements. This in turn can help us see what types of molecules will dissociate in water and become soluble. Compound bonded together ionically tend to be soluble in water due to them being salts. Those molecules fully di...

Sabrina Dunbar 1I

When the difference in electronegativity of two bonding elements is greater than 2, then the bond is going to be ionic in character. If the difference in electronegativity is less than 1.5, then the bond is going to be covalent. The above explanation did a good job explaining why this occurs.

Sabrina Dunbar 1I

When the electron jumps from the 2s orbital to the 2p orbital, it occupies a shell further from the nucleus, resulting in a higher atomic radius. It transforms to be more similar to the atoms in the p block that we know are larger than the s block atoms.

Sabrina Dunbar 1I

The number of valence electrons greatly affects the electron affinity of an atom. An atom is much more likely to gain an electron from another atom when their outer valence shell is almost filled because they are looking to reach the ideal state of having a full outer shell. Because the nuclear char...

Sabrina Dunbar 1I

In addition to the answer above, by looking at the Heisenberg equation that we have, we see that (deltaP)(deltaX)=h/4pi. deltaP and deltaX are inversely related, meaning that as one value increases, the other value decreases.

Sabrina Dunbar 1I

The two systems are actually very similar. For example, the p level has the capacity to hold 6 total electrons within it, and more specifically two per subset. There are three subsets within the p orbital, bringing the total amount of electrons tat can fit up to six which is what we would expect. Th...

Sabrina Dunbar 1I

Also, in lecture it was determined that the general rule of thumb number to be used to detect if the object has wavelike properties is determined by the value of lambda. Anything that has a wavelength less than 10^-15 generally does not have wavelike properties.

Sabrina Dunbar 1I

Hi Kelly, After finding the threshold energy in #33 to be 7.22x10^-19, you must look back at the equation hv(aka E)-phi=1/2mv^2. Phi is 7.22x10^-19 and because E=hc/lambda, you can calculate the energy of the photon to be 1.02x10^-18. Then, plug those two numbers into the above equation and you will...

Sabrina Dunbar 1I

Also be aware that the value in in kJ not just J, so you need to also do that conversion

Sabrina Dunbar 1I

The temp and pressure numbers are just denoting that the reaction is taking place at a standard temperature and pressure, meaning all of the products and reactants will behave ideally:) Long story short, the reaction is happening in a totally normal state so it can be solved without accounting for a...

Sabrina Dunbar 1I

I didn't use math to get O2 as the excess reactant, I just reasoned it out because the problem saw that if there is enough oxygen, the process can be further carried out in a secondary reaction, so thank you very much for elaborating on that! I would have been so lost on how to solve for it specific...

Search found 57 matches